The present invention relates to an imaging apparatus utilizing a nuclear magnetic resonance (NMR) phenomenon, and more particularly to a field gradient generator for generating a field gradient which changes at a high speed and has a large amplitude.
In an imaging apparatus utilizing an NMR phenomenon, it is necessary to separate and discriminate a signal from an object with a correspondence of the signal to a position in the object. There is one method in which position information is acquired by applying a field gradient to a magnetic field in which an object is placed, thereby producing a different resonance frequency or phase encoding amount. The basical principle of this method is disclosed by, for example, U.S. Pat. No. 4,506,222. Most of the NMR imaging apparatuses using this method and another position discriminating method include three sets of gradient coils which generate field gradients in three orthogonal directions respectively. The gradient coils are driven by respective independent drivers in predetermined sequences. JP-A-61-165648 discloses an example in which a multiple-coil arrangement is employed for a field gradient in a radial direction. An additional coil in the disclosed example is provided for eliminating higher order components of the magnetic field.
In the case where an object such as a heart involving any movement is to be examined, a large current must be applied to a coil in order to generate a large field gradient which changes at a high speed. This involves a problem that a component having a high withstanding or allowable operating voltage must be used for an FET, bipolar transistor or the like used as a driver. In an imaging method disclosed by U.S. Pat. No. 4,165,479, a field gradient in one direction is fixed and a field gradient in another direction is applied while the gradient direction is repetitively reversed. Where an image of a certain plane is required (planar imaging), one of field gradients to be subjected to reversals must be selected depending on the direction of that plane. Further, it is required that the field gradient to be subjected to reversals has a much higher changing rate and a much larger amplitude than the other field gradient.